The term die bonding describes the basic operation of attaching a semiconductor die to its package, or to some other substrate such as a tape carrier for tape automated bonding (TAB). First, the die is picked from either a diced wafer or a storage station, then it is aligned to a target pad on a carrier or substrate. It is then permanently attached to the substrate, usually by means of either a eutectic or epoxy bond.
For wire bonding, automated machinery connects a fine wire between pads on the die and terminals typically within the package. Many modern die bonders employ sophisticated positioning equipment together with a wide variety of optical-enhancement or machine-vision techniques to acquire and place the die, and to guide subsequent wire bonding operations.
One common wire bonding operation is referred to as "aluminum wedge bonding". A suitable apparatus for performing such bonding is bonder part number SWB-FA-US-30, available from Shinkawa Corporation of Japan. For purposes of the discussion that follows, the Shinkawa bonder is exemplary.
The bonder has a rotary table mounted with a chuck. The chuck holds the semiconductor package with the die already mounted therein. The rotary table includes a mechanism for raising the chuck, hence the die, to a predetermined height referred to as a "bonding level". A bonding tool places one end of an aluminum wire in contact with the pad (e.g.) to be bonded, and ultrasonic energy effects bonding of the wire to the pad. The bonding tool then moves to another location, and bonds another end of the wire to a package terminal (e.g.). In order to position the bonding tool at various locations in the plane of the die/package, either the rotary table is indexed about its axis, or the bonding tool itself is able to move in an X-Y direction. A mechanism is also provided for moving the bonding tool vertically, normal to the die plane, in the Z axis, so that the bonding tool can vary its height above the die between bonds.
During the bonding operation, the front surface of the die being bonded is required to be maintained at a fairly precise level (see point "P" in FIG. 2). This is due, in part, to a small depth of field in optical-enhancement or machine-vision apparatus associated with the bonder. The optimal vertical position of the die is established by selecting from a number of different height chucks. Thicker packages require shorter chucks, and vice-versa, and each chuck can accommodate only a narrow range of packages. (This assumes that the die thickness is constant.) In other words, changing from chuck-to-chuck is a rather coarse step function.
This limitation on vertical positioning of the die creates problems when a variety of package types having different package thicknesses are processed on the same bonder. Use of the "wrong" chuck may require the bonding tool to be lowered outside of its optimal range. Additionally, for each change is die or package thickness, the operator must re-focus the optical equipment. This represents non-productive use of available bonder time. While chucks having a variety of heights are supplied with the bonder, the need to constantly be selecting the proper chuck from a large assortment of chucks is in and of itself an additional nuisance and time-consuming task.